Method for processing signals produced by piezoelectric sensors mounted in a roadway for measuring the speed of vehicles

ABSTRACT

The invention relates to an analogue and digital processing method by sampling signals provided by two piezoelectric sensors C 1  and C 2,  making it possible to determine several speeds of a vehicle per axle.

[0001] The present invention relates to a method for analogue and digital processing of signals provided by piezoelectric sensors implanted in a roadway in order to allow the speed of vehicles passing over this road to be measured.

[0002] Techniques for producing piezoelectric sensors (French Patent Nos. 2703374, 2567550, etc.) and placement techniques are known and use coaxial sensors with ceramic isolation.

[0003] These sensors are coaxial linear sensors with a small diameter of between 1 and 8 mm.

[0004] Other sensors with plastic isolation (PVDF or piezopolymer) may also be used.

[0005] The aim of the present invention is to measure several speeds when a vehicle passes by, with at least two speed measurements for each wheel or each axle.

[0006] Multiple speed measurements have the advantage of enabling a mean speed for each axle, then a mean speed of the vehicle to be determined and of eliminating abnormal measurements.

[0007] The advantages of the method will become apparent from the following description.

[0008] The following figures are given by way of example, making it easier to understand the proposed invention.

[0009]FIGS. 1a and 1 b show the installation in the ground of two piezoelectric sensors perpendicular to the traffic and of an induction loop.

[0010]FIG. 1c shows the signals obtained on the induction loop detector and on the piezoelectric sensors with an impedance suitable for the problem.

[0011]FIGS. 2a and b show two sensors installed at an angle of 15 to 30° and the signal obtained when the wheels of a vehicle pass by.

[0012]FIG. 3 shows the force generated by a tyre on a road as a function of its longitudinal pressure.

[0013]FIG. 4 shows a tool allowing two sensors to be assembly parallel to each other and with a known separation.

[0014]FIG. 5 shows the signal obtained with an input impedance of 10 MΩ when the tyre of FIG. 3 passes over a sensor and its digitization.

[0015]FIG. 6 shows a vehicle with two axles passing over a group of two sensors installed with a known separation, and the measurement of the speed from characteristic points of curves with an input impedance of 10 MΩ.

[0016]FIG. 7 shows the same voltage variation obtained with an input impedance of about 40 to 100 kΩ and its digitization.

[0017]FIG. 8 shows a vehicle with two axles passing over a group of two sensors installed with a known separation, and the measurement of the speed from characteristic points of curves with an input impedance of 40 to 100 kΩ.

[0018]FIG. 9 shows an example of an electronic set-up according to the invention.

[0019]FIG. 10 shows a system using three sensors and one loop for the speed measurement.

[0020] French Patent No. 2673717 describes methods of processing the signal on input impedances of 10 MΩ, and of about 40 to 100 kΩ and explains the advantages and drawbacks thereof.

[0021] To carry out a correct speed measurement, the two sensors (1) and (2) described in FIG. 1b must be strictly parallel.

[0022] In order to do this, an assembly tool as described in FIG. 4 may be used; it will guarantee parallel assembly of the sensors (C1) and (C2), and a known separation (3) between the two sensors.

[0023] In this assembly, the sensors will be perfectly parallel in position C1 and C2 by machining four contact points (4), (5), (6) and (7) on the supports and with equal separations (8) and (9), these separations being controlled and measured on a 3D machine.

[0024] In this way, this separation between the two sensors will be known, fixed and independent of the region in which the vehicle will pass in the traffic lane.

[0025]FIG. 3 shows the distribution of vertical forces generated by a tyre in contact with the road.

[0026] First of all, we will examine the processing of the signal with an impedance of about 10 MΩ.

[0027]FIG. 5 represents the digitization of this signal when a wheel or when an axle passes over the sensor.

[0028] The time difference between two measurement points ΔT will be given by the scan speed of a clock of the computer or microprocessor system.

[0029] The negative part of the signal corresponds (see French Patent 2673717) to the deformation of the road due to the approach of the vehicle axle. The passage of the axle described in FIG. 3 will be physically embodied by inversion of the signal in the region (10), and by the appearance of a positive slope in the signal (ΔV/ΔT changes sign and becomes positive at (11)).

[0030] The appearance of the positive slope physically embodies the direct pressure of the tyre on the sensor. It will thus be possible to determine the start of the indentation due to the tyre on the sensor.

[0031] The inversion of the curve in the region (12) physically embodies the maximum vertical force passing over the piezoelectric sensor. It will thus be possible to determine this position of the axle corresponding to the axis of the axle (13).

[0032] Depending on the desired accuracy, this point could be physically embodied by inversion of the curve at the point (14) or by the intersection of the greatest positive and negative slopes determining a point (13).

[0033] It is therefore possible to determine a second characteristic position of the axle on the piezoelectric sensor.

[0034] It would also be possible to determine the point at which the indentation due to the tyre of the axle moves away from the piezoelectric sensor using the second inversion of the curve in the region (15). This point could be determined either by a whole period (16), or by the intersection of curves having the greatest slopes within a period ΔT of two scans given as the peak of the signal (17).

[0035] It can be seen from this description and from this drawing that it is possible to determine three characteristic instants for a tyre passing over a sensor:

[0036] start of the indentation due to the tyre (or due to the axle) on the sensor,

[0037] axis of maximum vertical force generated by the tyre on the sensor,

[0038] end of the indentation due to the tyre on the sensor.

[0039]FIG. 6 describes axles A, B, successively passing over each of the sensors (C1) and (C2) of the assembly described in FIG. 1b.

[0040] It is possible to determine three characteristic points, indexed 1, 2 and 3, for each axle (A) and (B) passing over each sensor (C1) and (C2). Since the separation between the two sensors is clearly known and measured, it is possible to determine for each axle the speed of displacement at the start of the indentation, the speed at the “axle centre” and the speed at the end of the indentation, for each axle between the two sensors (C1) and (C2). These speeds will be determined from the ratio of the separation of the sensors, divided by the times TA1, TA2 and TA3 for the first axle, by TB1, TB2 and TB3 for the second axle, and so on for each axle.

[0041] It can therefore be seen that it is possible to determine a maximum of six speeds for each vehicle with two axles. Vehicles with three axles demonstrate the determination of nine speeds, vehicles with four axles, twelve speeds, and so on and so forth.

[0042] The measurement of three speeds for the same axle must give substantially identical values in order to verify the homogeneity of the measurements. The difference between the speeds of two successive axles may be characteristic of an accelerating or decelerating vehicle.

[0043] At the same time as the speed, the system makes it possible to determine the dynamic weight of a vehicle and its category, as shown by patents such as French Patent 2673717 filed in March 1991.

[0044] A measurement of two parameters characterizing the speed at the start of the indentation and of the axes of the indentation may also be envisaged making it possible to obtain a minimum of four speed measurements per vehicle.

[0045] We will examine the case of processing the signal with an impedance of between 40 and 100 kΩ. This impedance, as described in prior French Patent No. 2673717, makes it possible to render the signal substantially symmetric and to overcome the effects of road flexibility on the shape of the signal.

[0046] On the other hand, it introduces a not insignificant time constant into the discharge of the piezoelectric sensor and into the asymptotic shape of the signal at the end of the passage of the axle. This deformation does not allow the position of the end of the indentation due to the tyre to be measured accurately.

[0047] The position at the start of the indentation due to the tyre will be determined as in the previous case, by the slope variation ΔV/ΔT at the start of the signal (region 18). The vertical force peak corresponding substantially to the axis of the axle (hereinafter called axle axis) results in inversion of the signal and in this signal passing through 0. The position of the axle axis will be determined by zero voltage of the signal. This position will be accurate since the signal variation is very sudden—region (19). The instant at which the indentation due to the axle moves away from the sensor is itself poorly determined because of the electrical constants of the unit formed by the sensor and the electronics.

[0048]FIG. 8 gives the characteristic points at the start of the indentation, indexed 1, and of the axle axis, indexed 2, for each axle A, B, etc. on each of sensors (C1) and (C2). Knowing the separation between the sensors makes it possible for the speed of each of the characteristic points of the axle to be easily calculated. The calculation is carried out by dividing the separation between the axis of the sensors by the times TA1 and TA2 for the first axle and TB1 and TB2 for the second axle.

[0049] It can be seen that it is possible to determine a minimum of two speeds per axle and therefore a minimum of four speeds for a vehicle with two axles.

[0050] As previously, a very small difference might be noticed between the speed of the two characteristic points of a given axle (verification of an abnormal value) while the difference in speed between two axles may indicate a variation in the speed of a vehicle (acceleration or braking).

[0051] It can be seen that, using these two different methods, it is possible to determine accurately the speed of a vehicle and any variation in its speed (acceleration or braking).

[0052] In the case of sensors inclined at an angle, as described in FIGS. 2a and 2 b, it will be possible to determine a series of speed measurements (2 or 3) for each wheel therefore between 8 and 12 measurements for a car or other vehicle with two axles.

[0053] The simultaneous determination of the category of the vehicle and of its weight may make it possible to introduce speed limits or warnings of speed limits being exceeded depending on the vehicle type.

[0054] This combination of properties may make it possible to trigger alarms or restraining measures. The device described above may also use a group of three parallel piezoelectric sensors combined with an induction loop (FIG. 10) or with another means of detecting the vehicle body.

[0055] The number of sensors may reach four or five or more.

[0056] In this last device, it is also possible to determine the speed of several characteristic points of each axle between the sensors (20) and (21), (21) and (22) and between the sensors (20) and (22). It will thus be possible to measure three speed groups per axle group of each vehicle.

[0057] The introduction of weather condition measurement may also make it possible to introduce variable speed thresholds depending on road conditions.

[0058] The electronic systems used are of the type already described in the prior patents and use operational amplifiers, 16- or 32-bit microprocessors, etc.

[0059]FIG. 9 is a possible and non-limiting exemplary embodiment of systems according to the invention.

[0060] The use of systems having the following characteristics:

[0061] separation between sensors of 1800 mm (3),

[0062] scan frequency giving a ΔT between two samples of 200 μs,

[0063] quartz-stabilized system,

[0064] 16-bit microprocessor with 12 bit converter, give tolerance intervals of ±1% ±½ significant digit, for example, 1% ±0.5 km/h for systems displaying km/h. 

1. Method of determining the speed of a vehicle passing over a road equipped with at least two piezoelectric sensors of coaxial linear type perpendicular to the traffic and parallel to each other, characterized in that the digital processing of the signals of each sensor by sampling makes it possible to determine the times when the start, the end and the pressure maximum generated by the indentations due to each axle or tyre pass over each sensor, and to calculate therefrom the speed of these three particular points between the successive sensors then to calculate therefrom the mean speed of each axle, that of the vehicle, its acceleration or its deceleration.
 2. Method as described in claim 1, where the input impedance of the piezoelectric sensors is greater than or equal to 10 MΩ.
 3. Method as described in claim 1, characterized in that the start, the end and the maximum level of pressure exerted by the tyre or the axle on the sensor are determined by successive inversions of the signal provided by the sensors.
 4. Method as described in claim 1, where the input impedance of the piezoelectric sensors is between 40 and 100 kΩ.
 5. Method as described in claim 1, characterized in that the start of the indentation due to the tyre or the axle is determined by the appearance of a positive slope in the signal, that the pressure maximum generated by the tyre or the axle is characterized by the signal passing through 0 after inversion.
 6. Method according to claim 1, characterized in that determining of the times at which the characteristic points of each tyre or of each axle pass over each sensor makes it possible to calculate up to three speeds for each axle.
 7. Method according to claim 1, characterized in that the sensors form an angle of 15 to 30° with respect to the traffic axis making it possible to distinguish each wheel and to determine the characteristic points of each wheel independently of each other and thus to determine up to a maximum of 12 speeds for a vehicle with two axles.
 8. Device according to the method described in claim 1 where the signal is digitized and where the particular times and speeds are determined by a computer or microprocessor.
 9. Method according to claim 7, characterized in that knowledge of multiple speeds for each axle makes it possible to eliminate abnormal value or values, to calculate the mean speed of the vehicle and the variations of this speed as a function of the axles. 